[Feature] Add setting to change the date the user started the grind

[carlssonemil]

Currently whenever the users tracks something in the app the current date will be stored in the beganGrind variable. Add a setting to change this date on the settings page as they might already have begun their progress before using this tracker (i.e. when the game was released).

[Voigtus]

Maybe we could add a field "started at release" and a second one "started at date" with a datepicker